Thermosetting resin compositions have been widely used as a variety of electrically insulating materials, structural materials, and adhesives by molding, casting, impregnating, laminating, and other forming techniques. Increasingly rigorous requirements are currently imposed in these applications, among which heat resistance is important.
Typical thermosetting resin compositions include polyimide and epoxy resins. The polyimide resins have satisfactory heat resistance, but are poorly workable in that they require prolonged heating at high temperatures for working. The epoxy resins modified to improve heat resistance are easily workable, but insufficient in mechanical and electrical properties at high temperatures, long-term thermal degradation resistance, and extreme heat resistant function.
Several replacements were proposed, for example, a thermosetting resin mixture comprising a polyimide and an alkenyl phenol or alkenyl phenol ether as disclosed in Japanese Patent Application KoKai (JP-A) No. 994/1977, and a heat resistant resin composition comprising a maleimide compound, a polyallylated phenol, and an epoxy resin as disclosed in Japanese Patent Publication No. 28416/1982. Since the polyallylated phenol used in the latter composition has a structure in which a polyallyl ether compound has been subject to Claisen rearrangement or Claisen rearrangement creates a phenolic hydroxyl group during heat curing, each substituting allyl group and a hydroxyl or ether group are at ortho-positions on a common aromatic ring and they would remain unreacted after curing especially in the case of novolak type resin compositions, leaving problems in cured properties and thermal degradation resistance at high temperatures.